What's correct is already answered but your other version "Das ist interessant nicht" sounds more like a question. You need to change a bit the punctuation like:"Das ist interessant, nicht (wahr)?"So "It's interesting, isn't it?". Otherwise, I don't know a real rule for where to place the "nicht" but sometimes I guess you can just place it by your feeling (or just by trying what you think is correct, considering also your English background)